1. Field of the Invention
The invention disclosed herein relates to imaging an earth formation from a borehole using acoustic waves.
2. Description of the Related Art
Exploration and production of hydrocarbons generally requires accurate measurements of earth formations, which may contain reservoirs of the hydrocarbons. Accurate measurements are important to enable an efficient use of exploration and production resources.
Well logging is a technique used to perform measurements of an earth formation from a borehole penetrating the formation. In well logging, a logging instrument is conveyed through the borehole. The logging instrument performs the measurements of the formation through the borehole. In one embodiment, a wireline is used to support the logging instrument and to transmit measurements to the surface of the earth for processing and recording.
Many types of measurements can be made of the earth formation. One type of measurement creates an image of the earth formation using acoustic waves. The acoustic waves travel from the instrument through a fluid in the borehole and into the earth formation. The earth formation in turn reflects some of the acoustic waves back to the instrument where the waves are recorded. In general, the intensity of acoustic waves reflected from a part of the formation is related to the material composing that particular part of the formation. Thus, the acoustic image can provide a visual indication of the various materials present in the formation.
In general, acoustic images having higher accuracy or higher resolution require acoustic waves with a higher frequency. However, as the frequency of the acoustic waves increase, the more attenuation of the acoustic waves by the borehole fluid occurs. Attenuation of acoustic waves causes weaker reflective waves and, therefore, a weaker acoustic measurement signal. The weaker measurement signal can result in a decrease in accuracy or resolution of the image. Thus, it appears that resolution of an acoustic image of an earth formation is limited by the attenuating characteristics of the borehole fluid.
Therefore, what are needed are techniques to increase the accuracy and resolution of acoustic images of an earth formation. Preferably, the techniques can be used in a borehole containing a fluid.